How do folds form in rock layers?

In geology, folds are undulations or waves in the Earth's crust that occur when originally horizontal strata are warped by various physical processes.1

Folds typically form through the following mechanisms:

• Compressional Forces: Long, linear folds common in mountainous regions are primarily attributed to compressional forces acting parallel to the Earth's surface and perpendicular to the fold axis.1
• Gravity: Some geologists suggest that strata may slide from vertically uplifted areas due to the influence of gravity, resulting in folding.1
• Glacial Pressure: The physical push exerted by an advancing glacier can deform weakly consolidated rocks into folds.1
• Compaction: The compaction of sedimentary rocks over buried hills can create gentle, localized folds.1

In nature, these features rarely result from a single process; rather, they are usually the product of a combination of these forces acting upon the rock layers. The ability of a rock to fold depends on its ductility, which allows it to undergo permanent deformation without fracturing.12